This invention relates to an improved channel electron multiplier made from a monolithic ceramic body and a method of making same. In particular it relates to a channel electron multiplier wherein said channel provides a preferably three dimensional, curved conduit for increased electron/wall collisions and for a device of smaller dimension, particularly when longer channel length is required. The invention further relates to phototubes employing those and similar electron multipliers, and to placement of the photoemission element relative to both the faceplate and passageway surface.
Electron multipliers are typically employed in multiplier phototubes where they serve as amplifiers of the current emitted from a photocathode when impinged upon by a light signal. In such prior art multiplier phototube devices, the photocathode, electron multiplier and other functional elements are enclosed as discrete elements in a surrounding vacuum envelope, for example an envelope made of glass. The vacuum environment inside the envelope is essentially stable and is controlled during the manufacture of the tube for optimum operational performance. The electron multiplier in this type of application generally employs a discrete metal alloy dynode such as formed from beryllium-copper or silver-magnesium alloys. Generally, the electron multiplier must be mounted as a discrete element within the envelope, and, as a result, the phototube device is susceptible to damage due to vibration and shock. Further, since the multiplier is wholly within the vacuum envelope, there is relatively poor thermal coupling between the hot dynode surfaces of the multiplier and the ambient external environment of the phototube.
There are other applications for electron multipliers that do not require a vacuum envelope. Such applications are, for example, in a mass spectrometer where ions are to be detected, and in an electron spectrometer where electrons are to be detected. In these applications the signal to be detected, i.e. ions or electrons, cannot penetrate the vacuum envelope but must instead impinge directly on the dynode surface of a "windowless" electron multiplier.
Electron multipliers with discrete metal alloy dynodes are not well suited for "windowless"applications in that secondary emission properties of their dynodes suffer adversely when exposed to the atmosphere. Furthermore, when the operating voltage is increased to compensate for the loss in secondary emission characteristics, the discrete dynode multiplier exhibits undesirable background signal (noise) due to field emission from the individual dynodes. For these reasons, a channel electron multiplier is often employed wherever "windowless"detection is required.
U.S. Pat. No. 3,128,408 to Goodrich et al discloses a channel multiplier device comprising a smooth glass tube having a straight axis with an internal semiconductor dynode surface layer which is most likely rich in silica and therefore a good secondary emitter. The "continuous" nature of said surface is less susceptible to extraneous field emissions, or noise, and can be exposed to the atmosphere without adversely effecting its secondary emitting properties.
Smooth glass tube channel electron multipliers have a relatively high negative temperature coefficient of resistivity (TCR) and a low thermal conductivity. Thus, they must have relatively high dynode resistance to avoid the creation of a condition known as "thermal runaway". This is a condition where, because of the low thermal conductivity of the glass channel electron multiplier, the ohmic heat of the dynode cannot be adequately conducted from the dynode, the dynode temperature continues to increase, causing further decrease in the dynode resistance until a catastrophic overheating occurs.
To avoid this problem, channel electron multipliers are manufactured with a relatively high dynode resistance. If the device is to be operable at elevated ambient temperature, the dynode resistance must be even higher. Consequently, the dynode bias current is limited to a low value (relative to discrete dynode multipliers) and its maximum signal is also limited proportionately. As a result, the channel multiplier frequently saturates at high signal levels and thus does not behave as a linear detector. It will be appreciated that ohmic heating of the dynode occurs as operating voltage is applied across the dynode. Because of the negative TCR, more electrical power is dissipated in the dynode, causing more ohmic heating and a further decrease in the dynode resistance.
In an effort to alleviate the deficiences of the typical glass tube channel multiplier, channel multipliers formed from ceramic supports have been developed. Such devices are exemplified in U.S. Pat. No. 3,244,922 to L G Wolfgang U.S. Pat. No. 4,095,132 to A. V. Fraioli and U.S. Pat. No. 3,612,946 to Toyoda.
As shown and described in U.S. Pat. Nos. 3,244,922 and 4,095,133, the electron multiplier is formed from two sections of ceramic material wherein a passageway or conduit is an elongated tube cut into at least one interior surface of the two ceramic sections. While such a channel can be curved as shown in the patent to Fraioli or undulating as shown in the patent to Wolfgang, each is limited to a two-dimensional configuration and thus may create only limited opportunities for electron/wall collisions.
In U.S. Pat. No. 3,612,946, a semi conducting ceramic material serves as the body and the dynode surface for the passage contained therein. For this device to function as an efficient channel electron multiplier, the direction of the longitudinal axis of its passage must essentially be parallel to the direction of current flow through the ceramic material, such a current flow resulting from the application of the electric potential required for operation.
The present invention is an improvement of the channel multiplier phototube devices of the prior art discussed above in that it combines the beneficial operation of the glass tube-type channel multiplier and the discrete dynode multiplier and adds a ruggedness and ease of manufacture heretofore unknown.
Accordingly, it is an object of the present invention to provide a channel electron multiplier phototube device which has a high gain with a minimum of background noise.
It is another object of the present invention to provide a phototube device including a channel multiplier having a dynode layer formed from a semiconducting material having good secondary emitting properties.
It is another object of the present invention to provide a phototube device including a channel multiplier having a 3-dimensional passageway therethrough so as to optimize electron/wall collisions and to provide for longer channels in a compact configuration.
It is another object of the present invention to provide a rugged, easily manufactured phototube device including a channel multiplier.
It is a further object of the present invention to provide a phototube device including a channel multiplier which can also serve as the insulating support for electrical leads, mounting brackets, aperture plates, photocathodes, signal anodes, and the like.
It is a further object of the present invention to provide a phototube device having an improved photocathode configuration.
The above and other objects and advantages of the invention will become more apparent in view of the following description in terms of the embodiments thereof which are shown in the accompanying drawings. It is to be understood, however, that the drawings are for illustration purposes only and not presented as a definition of the limits of the present invention.